Bridges

Photo by: OHRAUGE

Bridges are structures that provide a means of crossing natural barriers,
such as rivers, lakes, or gorges. Bridges are designed to carry railroad
cars, motor vehicles, or pedestrians. Bridges also support pipes, troughs,
or other conduits that transport materials, such as an oil pipeline or a
water aqueduct.

Humans have been constructing bridges since ancient times. The earliest
bridges were probably nothing more than felled trees used to cross rivers
or ditches. As civilization advanced, artisans discovered ways to use
stone, rock, mortar, and other natural materials to construct longer and
stronger bridges. Finally, as physicists and engineers began to develop
the principles underlying bridge construction, they incorporated other
materials such as iron, steel, and aluminum into the bridges they built.
There are four major types of bridges: beam, cantilever, arch, and
suspension.

Forces acting on a bridge

Three kinds of forces operate on any bridge: the dead load, the live load,
and the dynamic load. Dead load refers to the weight of the bridge itself.
Like any other structure, a bridge has a tendency to collapse simply
because of the gravitational forces acting on the materials of which the
bridge is made. Live load refers to traffic that moves across the bridge
as well as normal environmental factors such as changes in temperature,
precipitation, and winds. Dynamic load refers to environmental factors
that go beyond normal weather conditions, factors such as sudden gusts of
wind and earthquakes. All three factors must be taken into consideration
in the design of a bridge.

Words to Know

Bedrock:
Portion of Earth's mantle made of solid rock on which permanent
structures can be built.

Dead load:
The force exerted by a bridge as a result of its own weight.

Dynamic load:
The force exerted on a bridge as a result of unusual environmental
factors, such as earthquakes or strong gusts of wind.

Live load:
The force exerted on a bridge as a result of the traffic moving across
the bridge.

Piers:
Vertical columns, usually made of reinforced concrete or some other
strong material, on which bridges rest.

Suspenders:
Ropes or steel wires from which the roadway of a bridge is suspended.

Truss:
A structure that consists of a number of triangles joined to each
other.

Beam bridges

The simplest type of bridge consists of a single piece of material that
stretches from one side of a barrier to the other side. That piece of
material—called a beam or girder—rests directly on the
ground on each side or is supported on heavy foundations known as piers.
The length of
a beam bridge is limited by the weight of the beam itself plus the weight
of the traffic it carries. Longer beam bridges can be constructed by
joining a number of beams to each other in parallel sections.

Cantilever bridges

A cantilever bridge is a variation of the simple beam bridge. A cantilever
is a long arm that is anchored at one end and is free to move at the
opposite end. A diving board is an example of a cantilever. When anchored
firmly, a cantilever is a very strong structure. It consists of three
parts: the outer beams, the cantilevers, and the central beam. The
on-shore edge of the outer beam is attached to the ground itself or to a
pier (usually a vertical column of reinforced concrete) that is sunk into
the ground. The opposite edge of the outer beam is attached to a second
pier, sunk into the ground at some distance from the shore. Also attached
to the off-shore pier is one end of a cantilever. The free end of the
cantilever extends outward into the middle of the gap between the shores.
The cantilevers on either side of the gap are then joined by the central
beam.

Caisson

To build bridge piers, workers need a water-free environment to excavate
or dig the foundations. This is achieved by using a caisson, a hollow,
water-tight structure made of concrete, steel, or other material that
can be sunk into the ground. When building a bridge over a river,
workers sink a caisson filled with compressed air into the river until
it reaches the river bottom. The workers then go into the caisson and
dig out soil from the riverbed until they come to bedrock. The caisson,
which has sharp bottom edges, continually moves downward during the
digging until it comes to rest on bedrock. Concrete is then poured into
the caisson to form the lowest section of the new bridge pier.

Trusses.
The strength of a cantilever bridge (or any bridge) can be increased by
the use of trusses. A truss is structure that consists of a number of
triangles joined to each other. The triangle is an important component of
many kinds of structures because it is the only geometric figure that
cannot be pulled or pushed out of shape without changing the
length of one of its sides. The cantilever beam, end beams, and joining
beams in a cantilever bridge are often strengthened by adding trusses to
them. The trusses act somewhat like an extra panel of iron or steel,
adding strength to the bridge with relatively little additional weight.
The open structure of a truss also allows the wind to blow through them,
preventing additional stress on the bridge from this force.

Arch bridges

The main supporting structure in an arch bridge is one or more curved
elements. The dead and live forces that act on the arch bridge are
transmitted along the curved line of the arch into abutments or supporting
structures at either end. These abutments are sunk deep into the ground,
into bedrock if at all possible. They are, therefore, essentially
immovable and able to withstand very large forces exerted on the bridge
itself. This structure is so stable that piers are generally unnecessary
in an arch bridge.

The roadway of an arch bridge can be placed anywhere with relationship to
the arch: on top of it, beneath it, or somewhere within the arch. The
roadway is attached to the arch by vertical posts (ribs and columns)

A bowstring arch bridge in Arizona. The roadway is supported from the
arch by suspenders.
(Reproduced by permission of

JLM Visuals

.)

if the roadway is above the arch, by ropes or cables (suspenders) if the
roadway is below the arch, and by some combination of the two if the
roadway is somewhere within the arch.

Suspension bridges

In a suspension bridge, thick wire cables run across the top of at least
two towers and are anchored to the shorelines within heavy abutments. In
some cases, the roadway is supported directly by suspenders from the
cables. In other cases, the suspenders are attached to a truss, on top of
which the roadway is laid. In either case, the dead and light loads of the
bridge are transmitted to the cables which, in turn, exert stress on the
abutments. That stress is counteracted by attaching the abutments to
bedrock.

The towers in a suspension bridge typically rest on massive foundations
sunk deep into the riverbed or seabed beneath the bridge itself. The wire
cables that carry the weight of the bridge and its traffic are made of
parallel strands of steel wire woven together to make a single cable. Such
cables typically range in diameter from about 15 inches (38 centimeters)
to as much as 36 inches (91 centimeters).

Movable bridges

Traditionally, three kinds of movable bridges have been constructed over
waterways to allow the passage of boat traffic. In a swing bridge, the
roadway rotates around a central span, a large, heavy pier sunk into the
river bottom. In a bascule bridge, the roadway is raised like an ancient
drawbridge. It can be lifted either at one end or split in two halves in
the middle, each half rising in the opposite direction. In a vertical-lift
bridge, the whole central portion of the bridge is raised straight up by
means of steel ropes.

User Contributions:

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Thank you! This site was really helpful, much more so than the others that I have visited so far. I'm doing a project on bridge design and the defintions and information on cantilever bridges really helped me, hopefully I'll do well on it *crosses fingers*

In charge of a small NGO in Madagascar I need help to design a 360 meters long wire bridge to ferry small loads (250 - 300 kgs) such as systems used in Nepal.
Thanks for your assistance as I'm a bit lost,

This site was very useful as i am happening to be studying to build bridges myself. My only point would be about how you have said that a diving board is an example of a cantiliever bridge. I beg to differ, it is a common mistake though. Thank-you for your help,